Photodetectors such as APDs and PIN (p-intrinsic-n) photodiodes are employed for a wide variety of purposes. For example, an optical receiver in which a photodetector serves as a receiver element is one of the key element in an optical fiber transmission network. Optical receivers, in general, function to convert optical signals into electrical signals. A typical optical receiver includes a photodetector connected to the input of an amplifier (e.g., a transimpedance amplifier). The photodetector converts the optical signal it receives into an electric current that is supplied to the amplifier. The amplifier then generates at its output a voltage or current that is proportional to the electric current. With the recent spread of broadband networks, optical receivers (and optical transmitters) have increased in speed, typically increasing in bit rate from 1.25 Gbits/s to 2.5 Gbits/s. More recently still, bit rates up to 10 Gbits/s are beginning to be widely used.
As many systems lower bit rate systems are upgraded to high bit rates, one concern is the weak sensitivity of the optical receiver. To enhance receiver sensitivity APDs are often preferred because of their superior power sensitivity in comparison to PIN photodiodes. Unfortunately, an APD can be more difficult to tune and calibrate, in part because its avalanche multiplication factor varies with ambient temperature. The following will discuss in more detail the manner in which the dependency of the gain on the bias voltage of an APD varies with temperature.
When an APD is illuminated with light, its output current (i) vs. reverse voltage (v) can be shown by the upper curve in FIG. 1. As the reverse or bias voltage increases to the punch-through voltage (Vp), at which point the avalanche effect induces a step-up in the output current. The output continues to increase with bias voltage until avalanche breaks down at v=Vb. Of course, a dark current is generally always present at the output of the APD with or without illumination. Without illumination, the dark current rises with bias in accordance with the lower curve shown in FIG. 1.
In terms of the light power (P), APD responsivity (R) and avalanche gain (M), the output current generated by the APD can be expressed as:i=PRM  (1)
For v<Vp, there is no avalanche effect and the gain is M=1. Between Vp and Vb, the inverse gain (1/m) is a linear function with respect to the bias (v):1/M=(1−v/Vb)/M1 or M=M(v)=M1/(1−v/Vb)  (2)Where M1=M(0) is a constant.
From the data in FIG. 2 for an illustrative APD, 1/M1=0.9509 and 1/(M1Vb)=0.0302[V−1]. So, in this example M1=1.05 and Vb=31.5V. When biasing the illustrative APD to measure sensitivity, the avalanche gain is usually found to be optimized for avalanche gain values around M*=10, as shown in FIG. 3 for the illustrative APD at various temperatures.
The temperature dependences for the breakdown voltage (Vb), the bias for M=10 and the optimal bias for a representative APD available from OKI Semiconductor are shown in FIG. 4. Defined as the “percentage change per ° C.” from the breakdown voltage at 25° C. (Vb*), the temperature coefficient for Vb, ranges from 0.10 to 0.25%/° C. as listed in Table 1, which presents data available from an OKI Semiconductor datasheet. Also listed in Table 1 are the spreads for the breakdown voltage and reponsivities (M=1 and 10).
TABLE 1Specification of OKI's APDParameterSymbolTest ConditionsMin.Typ.Max.UnitWavelengthλ—1250—1620nmAPD Breakdown VoltageVBRID = 10 μA25° C.—3743V−40° C. to 85° C.—3750Temp. Coefficient of VBR**γ—0.100.150.25%/° C.APD ResponsivityRAPDλ = 1.55 μm, M = 10.80.9—A/Wλ = 1.31 μm, M = 10.750.85—λ = 1.62 μm, M = 1—0.75—ResponsivityRRL = 100 Ω, M = 10162638kV/WPin = −30 dBm, DifferentialBandwidthBWf-3 dB, RL = 50 Ω, M = 1017002000—MHzLow frequency cutofffc_lowRL = 50 Ω—3—kHzSensitivityPrmin2.488 Gbps,25° C.—−35−33.5dBmNRZ, Rext* = 10 dBBER = 10-ID,−40° C. to 85° C.—−33−31.5PRBS223−1,Rext* − 10 dBM = Mopt.OverloadPrmax2.488 Gbps, NRZ,−7−3—dBmBER = 10-ID, PRBS223−1,M = Mopt.Supply CurrentIccPin = 0 mW—4460mARecommended TIA Supply VoltageVcc—3.03.33.6V
In practice, due to variations of the breakdown voltage and the optimal value of the bias voltage, each APD chip or lot has to be tested for the optimal bias vs. temperature. The test data are programmed as a look-up table in the actual APD circuit, which also includes a temperature sensor. With a temperature sensor, an optimal bias is selected from the table to bias the APD for each discrete temperature range. This temperature characterization process can be costly to implement for most APD applications.